Joseph E. Armstrong

Annonaceae - Custard apples and Pawpaws

The Annonaceae is another systematically primitive family and is exclusively beetle pollinated. Previous field work in Costa Rica has documented insects using the extremely fleshy petals of Anaxagorea crassipetala (click to see a picture) as a brood substrate, a most limited and ephemeral resource. Use of the petals as a brood substrate by non-pollinators and a lack of reproductive success for both the tree (no successful pollinations) and the potential pollinators (no offspring produced on that brood substrate) suggest competitive exclusion.  Since the flowers are fleshy, open at first light, the coolest time each day, and use odor as an attractant, these flowers are likely to metabolically heat to aid fragrance diffusion. 

Fall 2000 Field Research in Costa Rica - To test these hypotheses, field experiments were conducted to  alter the competitive advantage of nonpollinators to deteremine if that alters the pollination success and the pollinator success since their reproductive activities are tied together.    Three ISU students and recent BS graduates spent 1-2 months as field research assistants (click to see students in rain forest).  A flower bagging treatment (click to see festive tagging of on an experimental tree) that prevented competitors from ovipositing in the petals prior to anthesis significantly increased fruit set, a three-fold increase.   However pollinating beetles did not brood in either control or treatment petals.   The bagging manipulation significantly reduced the reproductive success of nonpollinating, petal-brooding Dipterans.  When all the ovipositions were on the day of anthesis intraspecific competition was too severe for any to reach the pupal stage.  

What next?  Populations of small insects fluctuate and any particular study is like a snap shot in ecological time.  This experiment needs to be repeated to determine if the outcome is different under different conditions.  Larger trees were observed to have more fruit at the top of their crowns, so the interaction may be stratified.  Under story conditions may favor the dipterans in some way, so the experiment will be altered to take this into account.

Trees appeared to set more fruit on upper branches than on lower.  It is possible the interaction between fruit flies and mutualists is stronger lower in the understory, and this would mean taller, older trees might be more successful.  More experimentation is needed to test these ideas. 

Other projects waiting for field researchers.  Two species of Gaulteria exhibit no evidence of pollinators using the flowers as a brood substrate. Both genera provide floral visitors with nutritive reward and can be considered fruit mimics. But what costs and benefits are accrued by either delivering or deceiving pollinators searching for a brood substrate? Further trees flower in synchrony in a two day sequence of pollen-receiving and then pollen-dispersing, if so even-day trees swap pollen with odd-day trees, and vice versa.   The answers await future field work.